Various solid particle materials are subject to totally different treatment by many different industries. The suitable control of solid particle sizes might be important should the solid particles be within an aqueous mixture, an organic fluid mixture, or a dry mix of only solid particles.
In those industries which incorporate the treatment of a dry mixture of solid particles in their processes, the treatment of that dry mixture might be for two different purposes. For one purpose, a separation of a mixture of solid particles into numerous groups of particles with particles within each group being of a close range of particle size might be required. For another purpose, a treatment of the solid particle mixture might be intended to maintain a uniform mixture of the solid particles.
For example, in the coal industry, one step in the process of preparing coal for the various markets would be the separation, as by screening, of the coal into a series of particle size groups with each group comprising a chosen narrow range of particle size. The different screened sizes have been found most suitable for the various end uses. For domestic purposes, a large size range has been most beneficial. For some domestic purposes and some industrial purposes, a clean, graded coal of smaller size is preferred. Then, for carbonization, that is, the process of making coke, a still smaller size is preferred. Eventually, even the solid particles resulting from the process of preparing coke are separated into various particle sizes. Other industries give similar exemplary treatment to mixtures of solid particles.
If carried to an extreme, this type of treatment of a mixture of solid particles might be described as separating the particles into "homogeneous" portions.
In the second type of treatment of a mixture of solid particles referred to above, the purpose is to maintain, or develop, a uniform mixture of the solid particles in toto.
We are most knowledgeable about, and concerned with, the problem within the construction industry in handling, storing, and dispensing, crushed rock and sand.
As used in the construction industry, a supply of crushed rock, or other hard, inert material, of various sizes is generally referred to as an "aggregate". The attempt in the construction industry is always to try to maintain a "uniform mixture" of the aggregate, that is, an aggregate which has the various sized particles distributed evenly throughout the aggregate.
Aggregate material, crushed rock, gravel, or sand, is always accumulated by the supplier in extremely large quantities, as it is received in great quantities and dispensed in great quantities. In such great quantities, the consistent maintenance of a uniform mixture has always been difficult with the traditional means of handling or transferring the bulk material. For example, in the use of a conveyor system, the aggregate falls off the end of the conveyor and forms a conical pile, but the particles of the aggregate quickly separate according to their particle size, and the total aggregate is no longer a uniform mixture.
Typically, the supporting structure of the conveyor that is transferring this aggregate material remains stationary with the end of the conveyor positioned above a particular point while discharging the aggregate.
Aggregate suppliers and concrete suppliers store aggregate material in large piles as they receive it. Consequently, as described above, the aggregate material has become considerably non-uniform, and this non-uniformity usually affects the quality of the end product. In both the construction industry and the road building industry, the contractors want to build with concrete having a uniform aggregate throughout the concrete structure, not a concrete in one portion of the structure incorporating a limited range of small particles and in another area having a limited range of larger particles, although each might barely qualify for the construction according to contractual specifications.
The characteristics of a mixture of solid particles to separate according to size as we have described, is fully acknowledged in various industries. As a result, many attempts have been made to prevent this size segregation. The following patents which we have found in the prior art are examples of the recognition of this problem and the various methods employed in attempting to prevent size segregation:
______________________________________ U.S. Pat. No. 846,751 Melvin March 12, 1907 U.S. Pat. No. 936,182 Steidle Oct. 5, 1907 U.S. Pat. No. 1,663,173 Pioda March 20, 1928 U.S. Pat. No. 2,994,460 Matthews August 1, 1961 U.S. Pat. No. 4,264,213 Dillman April 28, 1981 ______________________________________